Webhooks send specific Event data. If you take a look at that development document, you will see that there are different sources that an event type can have. The target type would depend on the event type and are what would be considered the focus (or target) of a given event type. These corresponding values can be seen in the table as well. 

--Justin

Yes, but are all of the properties in the event data documented somewhere? How can I understand what each property is, and can be?

We provide an example Event payload here as a response from the Get Event endpoint: 

https://dev.frontapp.com/reference/get_events-event-id

We’ve also flagged the docs for improvement

This is super helpful, thanks :)